Method for the preparation of a planographic printing master

ABSTRACT

AN IMPROVEMENT ON U.S.P. 3,476,937 WHEREIN SILVER HALIDE GRAINS OF THE INTERNAL IMAGE-FORMING TYPE ARE DIRECTLY INCORPORATED IN THE HEAT-SENSITIVE LAYER OF THE BASIC PATENT FORMED BY NORMALLY SOLID THERMOPLASTIC POLYMER PARTICLES DISPERSED INA HYDROPHILIC BINDER SO THAT BY EXPOSING THE RESULTANT LAYER TO A LIGHT IMAGE, PHOTOGRAPHICALLY DEVELOPING THE EXPOSED LAYER WITH A PHOTOGRAPHIC DEVELOPING LAYER TO OBTAIN A DIRECT-POSITIVE SILVER IMAGE HAVING A DENSITY OF AT LEAST 0.1 ABOVE BACKGROUND, AND UNIFORMLY EXPOSING THE DEVELOPED LAYER TO RADIATION ABSORBED BY THE SILVER IMAGE AND CONVERTED INTO HEAT, SUFFICIENT HYDROPHOBIC/HYDROPHILIC DIFFERENTIATION IS OBTAINED AS TO ADAPT THE RESULTANT MATERIAL FOR USE AS A PLANOGRAPHIC PRINTING MASTER.

United States Patent 3,672,892 Patented June 27, 1972 hoe US. CI. 96-33 12 Claims ABSTRACT OF THE DISCLOSURE An improvement on U.S.P. 3,476,937 wherein silver halide grains of the internal image-forming type are directly incorporated in the heat-sensitive layer of the basic patent formed by normally solid thermoplastic polymer particles dispersed in a hydrophilic binder so that by exposing the resultant layer to a light image, photographically developing the exposed layer with a photographic developing layer to obtain a direct-positive silver image having a density of at least 0.1 above background, and uniformly exposing the developed layer to radiation absorbed by the silver image and converted into heat, sufficient hydrophobic/hydrophilic differentiation is obtained as to adapt the resultant material for use as a planographic printing master.

The present invention relates to the preparation of a printing master for use in planographic printing and to materials suited therefor.

Planographic printing is based on the physical property of repellence of greasy materials to water. In planographic printing the printing master surface contains the pattern of the image to be printed in terms of a differentiation in water repellency. The printing plate is usually prepared by image-wise aflixing a water-repellent substance or composition, usually greasy, resinous or waxy in nature, to a hydrophilic surface. According to a more recent technique a hydrophilic surface is image-wise converted into a hydrophobic one, by heat-absorption as is described e.g. in the Belgian patent specifications 656,713 filed Dec. 7, 1964 by Gevaert Photo-Producten N.V. and 681,138 filed May 17, 1966 by Gevaert-Agfa N.V.

In common planographic printing use is made of inks that are composed of a lipophilic phase, wherein pigments and other suitable ink ingredients are worked up, which does not absorb water or only to a small extent. During printing according to the classical planographic process the printing plate is kept supplied with an aqueous composition so as to keep the non-printing areas sufiiciently hydrophilic. This method of printing thus requires in addition to an inking system a so-called damping system by means of which the hydrophilic areas are covered with water or a colourless aqueous composition and thus kept grease-repellent.

According to a more recently developed planographic printing process, called reversed planographic printing process use is made of a printing master, wherein the hydrophilic parts constitute the printing parts. The printing ink used in this process contains an aqueous ink composition as coloured medium and the damping system is used for applying a colourless oleophilic liquid composition. This printing process is very suitable for direct planographic printing, in other words a process wherein there is no need to reverse the image (offset) by means of an intermediate cylinder with rubber blanket.

Various proposals have been made to record printed matter by irradiating the printed markings of an original so that they become heated while in heat-conductive relationship to a heat-sensitive recording material. Certain proposals have also been made to record a pattern of light by exposing to it a recording material, which is susceptible to heating and undergoes change in the illuminated areas due to internal absorption of light and conversion thereof to heat energy.

A recording technique wherein a hydrophilic recording material is susceptible to heating and wherein heated portions of a heat-sensitive layer become hydrophobic is described in the Belgian patent specifications 656,713 filed Dec. 7, 1964 by Gevaert Photo-Producteu N.V. and 618,138 filed May 17, 1966 by Gevaert-Agfa N.V. which correspond to U.S.P. 3,476,937, issued Nov. 4, 196 9. The hydrophilic-hydrophobic differentiation obtained according to the technique described and claimed in said specifications is sufiicient for use of the resultant materials in the preparation of planographic printing masters.

The present invention is concerned with recording materials which are attractive from the standpoint of manufacture and processing and are suited for recording as described in the above Belgian patent specifications, and contributes to the preparation of planographic printing masters for planographic printing on an economic commercial scale.

There has been found now a method for producing a planographic printing master comprising the steps of:

(a) Image-wise exposing to light a recording material comprising a layer containing a light-sensitive silver halide emulsion, wherein mainly an internal latent image and a minor external latent image as defined in the description can be formed and further comprising in a hydrophilic heat-sensitive surface layer in heat-conductive relationship with said silver halide, and consequently with the silver formed thereof, hydrophobic thermoplastic particles solid at room temperature which particles are dispersed in a hydrophilic binder, prefereably in a ratio by weight greater than 1:1;

(b) Developing the exposed silver halide emulsion in such away that a photographic direct-positive silver image is obtained, the density of which is at least 0.1 above the density in the non-image areas, and

(0) Overall exposing the recording material to electromagnetic radiation, which is absorbed by the silver image and transformed into heat and by said heat produces in the heat-sensitive surface layer the image-wise hydrophilic hydrphobic differentiation necessary for making possible planographic printing.

According to the most interesting embodiment of the present invention the silver halide is contained in the heatsensitive surface layer so that the hydrophilic binder contained therein is common to the silver halide grains and the hydrophobic thermoplastic polymer particles.

In a preferred embodiment the heat-sensitive surface layer contains hydrophobic thermoplastic polymer particles, vnown as latex particles, but of the type solid at room temperature. The ratio by weight of said particles with respect to the hydrophilic binder should preferably be in excess of 3:2. The heat-sensitive surface layer more preferably contains said thermoplastic particles in a ratio by weight of 25:10 to 30:10 to the hydrophilic binder. The amount of silver halide relative to the thermoplastic polymer particles is preferably not very much lower than the equivalent to 1 g. of silver per 5 g. of said polymer particles. The heat-sensitive surface layer preferably contains at least 50% by volume of the dispersion of said polymer particles in the hydrophilic binder.

Latex compositions and hydrophilic binders suited for use in the heat-sensitive surface layer of the recording material of the present invention are described in the Belgian patent specifications 656,713 filed Dec. 7, 1964 by Gevaert Photo-Producten N.V. and 681,138 filed May 17, 1966 by Gevaert-Agfa N.V.

The particle size of the hydrophobic dispersed material preferably ranges from 0.01 to 50 In the preparation of the hydrophilic surface layer the thermoplastic hydrophobic particles, which are solid at room temperature, are dispersed in an aqueous medium preferably already containing the silver halide emulsion, and the dispersion thus obtained is preferably coated at a temperature not substantially higher than room temperature. The latex particles are surrounded by a wetting or dispersing agent since in the preparation of a latex such a wetting or dispersing agent is always used for dispersing the monomer. The latex particles preferably soften between 10 and 200 C. above room temperature. As examples of suitable latex polymers may be cited such polymers having a melting point or a glass-transition temperature between 10 and 200 C. above room temperature. Particularly suitable polymers are poly(ethylene), and poly(vinylidene chloride) having a melting point of 110 and 190 C. respectively, and the following polymers with their respective glass-transition temperatures: poly(styrene) (100 C.), poly(methyl methacrylate) (between 70 and 105 C.), poly(vinyl chloride) (near 70 C.), poly- (acrylonitrile) near 100 C.), poly (N-vinyl carbazole) (200 C.).

Particulars about the preparation of hydrophobic thermoplastic polymer particles in latex form can be found in the Belgian patent specification 681,138 filed May 17, 1966 by Gevaert-Agfa N.V.

In the hydrophobic dispersed phase of the surface layer use can be made of hydrophobic solid substances alone or together with the latex particles, said hydrophobic solid substances being of a wax-like nature or consistency.

Materials making up the continuous phase of the heatsensitive surface layer used in the present invention are hydrophilic binding agents e.g. hydrophilic natural colloids, modified hydrophilic natural colloids, or synthetic hydrophilic polymers. More particularly they may be selected from such film-forming natural or modified natural hydrophilic colloids, which do not adversely affect the photographic properties of the silver halide e.g. gelatin, glue, casein, zein, hydroxyethylcellulose, carboxymethylcellulose, gum arabic; sodium alginate, and hydrophilic derivatives of such colloids. They may also be selected from such synthetic hydrophilic polymers, as e.g. poly(vinyl alcohol), poly(vinyl pyrrolidone), poly(vinyl amine), poly(ethylene oxide), poly(styrene sulphonic acid), polyacrylic acid and hydrophilic copolymers and derivatives of such polymers. In preparing the silver halide emulsion gelatin is used preferably.

The hydrophilic binder material used in the heat-sensitive surface layer is preferably hardened to some extent to obtain a layer with higher mechanical strength and resistance to wear during printing. Thus, e.g., a hydrophilic binding agent of the gelatin type may be hardened by reaction with an aldehyde such as formaldehyde or glyoxal. The hardening agents may be incorporated into the coating composition or they may be incorporated into the coated layer by imbibition. The use of latent hardening agents, from which e.g. the hardening agent is generated by heating is not excluded.

The amount and type of hardening agent is a function of the mechanical strength desired. When using gelatin the amount of hardening agent may be in the range of 0.5 to by weight in respect of the gelatin. The hardening of the recording layer may be effected with a solution containing the hardening agent in an amount of 2 to 25% by weight.

The support of the recording material should impart sufiicient mechanical strength to the material as needed for printing and may be a paper support, a metal sheet or a synthetic film support that is either transparent or not. The silver halide emulsion layer must strongly adhere to the support for not getting loose therefrom during printing. If necessary one or more appropriate subbing layers are applied between the support and the silver halide emulsion layer.

G. p-Hydroxyphenylglycine 10 Sodium carbonate (cryst.)

Water to 1000 ccs.

less grains are developable than in a developing solution which acts as a developer for latent image inside the grains i.e. a so-called internal developer such as the following developing solution:

Hydroquinone .v. 15 Monomethyl-p-aminophenol sulphate 15 Sodium sulphite (anhydrous) 50 Potassium bromide 10 Sodium hydroxide 25 Sodium thiosulphate (cryst.) 20

Water to 1000 ccs.

For convenience, the emulsion of the type just described will be referred to in the claims as an emulsion of the internal type.

The difference in density between the areas in which the direct-positive image has been developed and the background areas should amount to at least 0.1 in order to obtain a hydrophilic-hydrophobic differentiation in the recording material by overall exposing it to electromagnetic radiation, which by absorption by the silver image is converted into heat. So a minimum difference in density (only 0.1) already makes possible the achievement of a hydrophilic-hydrophobic differentiation according to the method of the present invention. The density of the direct-positive image is preferably at least 0.3 and the fog in the non-image areas preferably does not exceed 0.5, the density of the positive image exceeding the density of the fog with at least 0.1.

In practice, said difference in density is generally much larger than 0.1. Of course, this offers the advantage of leaving a greater latitude for the overall exposure step, which follows the development step of the directpositive image, i.e. the overally exposure step becomes less critical.

Silver halide emulsions that meet the above requirement and consequently are suitable for being employed in the recording materials and the method of the invention are generally not or only slightly chemically ripened silver halide emulsions, since the extent of the surface latent image forming ability increases with the degree of chemical ripening.

Silver chlorobromide emulsions comprising at least 20 mole percent of silver bromide and pure silver bromide emulsions have proved to be very suitable. When these emulsions comprise a certain amount of silver iodide, often not more than 5 moles percent relative to the total amount of silver halide, they are just as well suitable. Washed as well as unwashed silver halide emulsions may be used.

The silver halide emulsions for use in preparing the recording material and carry out the method of the present invention are generally gelatin silver halide emulsions. However, the gelatin may at least partly be replaced e.g. by another protein, a hydrophilic non-proteinaceous colloid, e.g. a poly(vinyl pyrrolidone) or a synthetic polymeric substance applied from an aqueous dispersion, i.e. a latex e.g. a poly(ethyl acrylate) latex. The presence of such other binders often has also a favourable photographic effect on the formation of the direct-positive image. For instance, the addition of the ply(vinyl pyrrolidone) and of said poly (ethyl acrylate) latex in most cases increases the maximum density of the direct-positive image. For other suitable binders reference is made to the hydrophilic colloids mentioned above for use in the heatsensitive surface layer.

The light-sensitive silver halide emulsions may be prepared according to all known and conventional techniques for emulsion preparation. A method according to which emulsions are prepared, which have proved to be particularly suitable for the purpose of the invention, is the socalled conversion method. According tothis method a more soluble silver halide is converted into a less soluble silver halide e.g. a silver chloride emulsion is converted in the presence of water-soluble bromide and occasionally iodide, the amounts of which are adapted in view of the final composition aimed at, into a silver chlorobromide or a silver bromide emulsion, which may occasionally comprise small amounts of silver iodide. This conversion is carried out preferably very slowly by several consecutive steps. Another technique according to which emulsions with an increased internal latent image sensitivity may be obtained is described in the British patent specification 1,011,062 filed May 15, 1962 by Kodak Ltd. For further specifications of the nature and preparation of light-sensitive silver halide emulsion layers suitable for use according to the present invention we refer to the specific examples given below.

A considerable increase in density of the direct-positive image can be obtained by incorporating at least one compound that sets free iodide ions in aqueous medium and/ or at least one compound that sets free bromide ions in aqueous medium into the material in effective contact with the silver halide emulsion layer. By in efiective contact there is meant that the said compounds are provided on a place from which they can act upon the silver halide emulsion layer at the appropriate moment i.e. during the development. They may be incorporated into whatever composite layer of the material that is in water-permeable relationship with the light-sensitive silver halide emulsion layer; however, they are preferably present in the lightsensitive silver halide emulsion layer itself. They may be incorporated into the material by soaking the latter be fore as well as after exposure in an aqueous composition containing them (this aqueous composition may be the developer for producing the direct-positive image itself), or by adding them to the coating composition of a particular layer of the recording material. As said above the compounds that set free iodide ions or bromide ions are preferably incorporated into the light-sensitive silver halide emulsion layer itself. They are generally added to the light-sensitive silver halide emulsion, after the silver halide has been precipitated and occasionally converted.

The compounds that set free iodide ions or bromide ions can be applied in greatly varying concentrations i.e. in concentrations that depend on the nature of the compounds used and of the light-sensitive silver halide emulsion. However, their concentration usually varies between 0.01 and 20 g. preferably between 0.1 and g., per mole of silver halide.

Suitable compounds setting free iodide ions are among others water-soluble iodides, inorganic as well as organic iodides, organic compounds with labile iodine atom, onium chloroiodates, and molecular iodine and its addition products, e.g. with poly(vinyl pyrrolidone), with polyoxyalkylenes and their derivatives, or with quaternary ammonium compounds.

Suitable inorganic iodides are for instance, calcium iodide, ammonium iodide, lithium iodide, magnesium iodide, potassium iodide, sodium iodide, barium iodide, cadmium iodide, and zinc iodide.

Suitable organic iodides are for instance the iodides having the following structural formulae:

0 H3 uacm ona (3H tetramethylammonium iodide,

tetraethylammonium iodide-3diiodine,

CH mo-wmnmhiatm] r- (3H3 1,1, 1-dodecyl-dimethyl-hydrazinium( 1 iodide,

OH CH3 l-methyl-8-hydroxy-quinolinium iodide,

1-methyl-2-iodo-quinolinium iodide,

1,2,3,4-tetrahydro-8 hydroxy-1,1-dimethyl-quinolinium 3,5-dimorpholino-dithiolium iodide, and

diphenyl-iodonium iodide.

Organic compounds with labile iodine atom, which have proved to be usitable for use according to the invention,

are for instance mono-iodo-acetic acid and 4-indo-butane sulphonic acid potassium salt.

Finally, as regards the onium chloroiodates there can be referred to Belgian patent specification 515,895 filed Nov.

29, 1952 by Gevaert Photo-'Producten N.V.; examples of suitable onium chloroiodates are among others:

trimethyKo-methoxycafibonyl)-aniliniu-m dichloroiodate, and

benzyl-triphenyl-phosphonium dichloroiodate.

Suitable compounds setting free bromide ions are among others inorganic and organic bromides such as ammonium bromide, lithium bromide, sodium bromide, potassium bromide, magnesium bromide, calcium bromide, barioum bromide, cadmium bromide, and zinc bromide as inorganic bromides and tetraethylammonium bromide and ethylpyriddinium bromide as organic bromides. Organic compounds with labile bromine atom such as monobromoacetic acid are also suitable for being used in the present process as compounds setting free bromide ions.

In a first step for the preparation of the planographic printing master the recording material is image-wise exposed to the original to be reproduced. Thus, the recording material can be exposed to a halftone original (line or screen original) or to a continuous tone original together with an optical or contact screen in order to produce a halftone print.

The image-wise exposure can be adapted to the desired result. Thus the exposure can be either a high-intensity exposure such as flash exposure or a normal-intensity exposure such as daylight exposure as well as a low-intensity exposure such as an exposure by means of a printer or an exposure of still lower intensity, the exposure-time being dependent on the sensitivity of the light-sensitive material for the direct-positive image formation, which sensitivity according to the present invention may be a camera speed as well as a lower sensitivity.

In a further step a direct-positive image is produced by developing the recording material with an energetic surface developer, for instance by conducting the material through a tray containing such developer by means of a lick-roller, by spraying or by rubbing. The developer may be more or less viscous. It must be a surface developer i.e. a deevloper that contains no or at least no eflective amount of solvents for silver halide. By the expression solvents for silver halide particularly strong solvents for silver halide such as water-soluble thiocyanates, thiosulphates, ammonia, etc. are meant. Indeed, compounds, which may be considered as but very weak solvents for silver halide, such as sodium sulphite for the case the silver halide is' silver bromide, silver chlorobromide or silver bromoiodide, may be present in the developer.

Further the developer must be an energetic developer. The high energy required can be realized by taking for instance one or more of the following measures:

using a developer with relatively high pH value,

working at an elevated temperature,

using a developer whereinthe concentration of the various compounds present is relatively high,

incorporating developing activators such as cyclohexylamine, phenylhydrazine, polyethylene glycol and derivatives in the developer, and

omitting, at least for the greater part, restrainers such as potassium bromide from the developer.

The choice of the developing substances too determines the energy of the developer. Examples of suitable developing substances are among others the combination of 1- phenyl-3-pyrazolidinone with hydroquinone and the combination of monomethyl-p-aminophenol sulphate with hydroquinone. In general there may be said that the more energetic the developer, the better the result obtained. A part or the total amount of the developing compound(s) may be incorporated into the recording material so that they stand or can come in effective contact with the silver halide grains during the development stage of the direct positive image. In this way, the developer may be reduced to an alkaline composition that is substantially free from developing compound(s), i.e. a so-called activator solution.

While the image-wise exposed light-sensitive layer is present in the developer or is moistened therewith in some other way the said light-sensitive material is exposed uniformly to actinic light of low intensity. This overall exposure may begin simultaneously with the treatment of the light-sensitive material with developer but preferably occurs somewhat later, e.g. from 5 to 30 seconds later. The duration of the said secondary or overall exposure is not very critical and may vary between 10 seconds and several minutes according to the nature of the light-sensitive material, the composition, and the temperature of the developer, the intensity of the light-source employed, etc. In many cases the said overall-exposure lasts as long as the development. This offers the additional advantage that one can follow the development and stop it when the direct positive image formation is throught to have reached a suflicient density for carrying out the further processing steps of the method according to the invention.

The density of the direct positive image can be made I ing in and dodging. The uniformity of said overal exposure to actinic light of low intensity is mostly achieved by interposing between the light-source and the light-sensitive material to be exposed a light-diffusing member that transmits at least part of the light rays oft he spectral region to which the light-sensitive material is sensitive. The required low intensity of the light rays, which reach the light-sensitive material, can be obtained by adapting the light-source and/or the light-diffusing member.

:An absolute maximum value for the intensity of the said secondary or overall exposure can hardly be given since said exposure is strongly dependent on the nature of the light-sensitive material used. In general flash exposure and normal daylight exposure are of too high an intensity.

A suitable processing device for producing the directpositive image obtained as above mentioned is described and claimed in the Belgian patent specification 703,144 filed Aug. 28, 1967 by Gevaert-Agfa N .V.

Instead of or in addition to the overall exposure at least one fogging agent selected from hydrazine, carbazic acid, bicarbamic acid, and/or at least one water-soluble salt thereof may be added to the energetic surface developer and/or to a preceding processing liquid for the light-sensitive recording material. These fogging agents cause the formation of a latent direct-positive image that will be developable by the energetic surface developer. The best results are obtained with the fogging agent(s) added to the developer itself. When using in this embodimerit of the present invention another processing liquid incorporating the fogging agent(s), this liquid is preferably employed just before the development step. The pH of the liquid, to which the fogging agent has been added is preferably fairly high and in most cases amounts to at least about 10. A preceding separate processing liquid containing the fogging agent(s) may comprise other ingredients such as one or more of the ingredients that otherwise are incorporated into the developer.

'Carbazic acid and its water-soluble salts can be prepared e.g. as described in Berichte 37, 4523 (1904). Examples of water-soluble salts of carbazic acid are, the hydrazine salt of carbazic acid and the sodium salt of carbazic acid. An example of a water-soluble salt of bicarbamic acid is the potassium salt of bicarbamic acid. The terms carbazic acid and bicarbamic acid include the substituted compounds too.

The fogging agent is added usually to the processing liquid in an amount varying between about 1 g. and about 50 g. preferably between g. and 20 g. per litre.

For the purpose of the invention, a complete development is not absolutely necessary and so the development can be stopped already at the moment a sufficient density difierence is reached, thus before the density of the direct-positive image has reached its highest value.

In general a sufficient development of the direct positive image does not take much time and in most cases can be carried out in from to 60".

The minimum density of the direct positive image can still be lowered by incorporating a fog-inhibiting compound into the light-sensitive recording material. This fog-inhibiting compound must be present in effective contact with the silver halide emulsion layer. By in effective contact is meant that the fog-inhibiting compound is provided in the light-sensitive recording material on a place from which it can act upon the silver halide emulsion layer at the appropriate moment, in this case during the soaking of the light-sensitive material with the developer. The fog-inhibiting compound is preferably incorporated in the silver halide emulsion layer itself, but may also be present in an other water-permeable non-lightsensitive layer of the light-sensitive material such as a covering layer or an intermediate layer, from which it is dissolved by the developer liquid and diffuses to the light-sensitive layer. It is even possible to incorporate considerable amounts of the fog-inhibiting compound into the developer liquid.

The fog-inhibiting compounds give rise to a lowering of the minimum density of the direct-positive image even when present only in small amounts. They are sufficiently efficient when applied in the common fog-inhibiting amounts. The best results i.e. the most clear minimum densities are attained, however, when using the fog-inhibiting compounds in amounts larger than the common fog-inhibiting amounts i.e. in amounts that normally are not applied to light-sensitive materials since with such amounts the light-sensitive material would be desensitized too much. Surprisingly, these large amounts of fog-inhibiting compound do not lower the direct-positive image sensitivity of the light-sensitive recording material but in most cases even enhance it and also do not prevent the formation of a direct-positive image with good maximum density. Specific data about the concentrations in which the fog-inhibiting compounds are generally employed and yield optimal results can hardly be given since these concentrations are widely divergent according to the nature of the fog-inhibiting compound and the kind of silver halide emulsion layer used. For instance in the case of a silver bromide emulsion layer considerably less fog-inhibiting compound must be used than in the case of a silver chlorobromide emulsion layer for attaining almost equivalent results.

Fog-inhibiting compounds of the following classes have proved to be particularly suitable for being applied in this embodiment of the invention:

The class of the heterocyclic thione compounds such as:

4-pheny1-A -l,2,4-triazoline-S-thione, 3-phenyl-A -1,2,4-triazoline-5 -thione, -1-methyl-2-tetrazoline-S-thione, 1-phenyl-2-tetrazoline-S-thione,

ll-naphthyl) -2-tetrazoline-5-thione, l-(Z-naphthyl)-2-tetrazoline-5-thione,

1 0 1- (9-anthryl) -2-tetrazoline-5-thione, .1-( 3,4-dichlorophenyl) -2-tetrazoline-5-thione, 1- o-meth'oxy-phenyl -2-tetrazoline-5-thione, 1- o-biphenyl -2-tetrazoline-5-thione, l- (p-biphenyl) -2-tetrazoline-5-thione, 1-(2-naphthyl) -4,4,-6-trimethyl-1,2,3,4-tetrahydropyrimidine-Z-thione;

The class of the aromatic and aliphatic mercapto compounds such as:

Z-mercapto-ethyl carbanilate;

T he class of the benzotriazoles such as:

5-(3-phenylureido) -l'H-benzotria'Zole; and

The class of the fused oxo-compounds containing at least two nitrogen atoms such as: 5-nonyl-7-oxo-4,7-dihydro-s-triazolo-[l,5-a]-pyridine, and

pyrimidine, 2-methy1-4-hydroxypyrimido- 1,2-a] -benzirnidazole. I

It will be understood that these classes are intended to refer also to the tautomeric structures of said compounds aud to salts of said compounds or occasionally of said tautomeric structures. Mixtures of two or more fog-inhibiting compounds may also be employed.

In the method according to the invention the maximum or minimum density of the direct-positive image can still be improved and the characteristics of the light-sensitive recording material can be altered by using all kinds of ingredients which are generally known in the art of emulsion preparation and some of which will be set out specifically hereinafter.

Most of these ingredients are preferably incorporated into the light-sensitive recording material itself, in eflfective contact with the silver halide emulsion layer and favourably in the latter layer itself. Many of these ingredients, however, may be incorporated into the energetic surface developer with the same favourable result. Combinations of two or more of said ingredients, of course, may also be used.

Among the said ingredients may be mentioned some binders for at least partially replacing gelatin as a binder for the silver halide grains and referred to already above in the description, further optical sensitizers and moistening agents such as polyalkylene glycols, i.a. polyethyleneglycols, sulphonated fatty acids, saponine and the like. The presence of optical sensitizers and of polyalkylene glycols in the silver halide emulsion layer in most cases has a favourable effect on the maximum density of the direct-positive image.

The development of the direct-image may be stopped in a stop-bath, whereupon the photographic direct-positive image may be stabilized or fixed e.g. in a usual fixing solution containing thiosulphate ions. Indeed, neither, the stopping step nor fixing step are essential for the purpose of the present invention since before the image differentiation is lost by a complete blackening the heating step can take place and effect the image-wise hydrophobic-hydrophilic image differentiation.

Depending on the sensitivity of the silver halide emulsion the treatment of this emulsion after image-wise exposure and development may occur in full daylight or dimmed daylight.

In order to produce the desired image-wise hydrophobic-hydrophilic differentiation the developed directpositive silver halide emulsion containing the direct-positive silver image is overall exposed to electromagnetic radiation, which is absorbed in the silver image and transformed therein into heat. The said exposure and heattransformation are of a suflicient degree to selectively effect the change of the hydrophilic character of the heatsensitive surface into a hydrophobic character.

While this overall exposure is carried out the recording material may not be dripping wet but moist. Self-evidently, the ommission of the thorough drying is of interest for a rapid and economic processing.

The overall exposure for heating the recording material, in which the silver image has already been produced, is preferably carried out with visible light and/or infrared radiation which light and/or radiation are transformed into heat in the silver metal of the image.

In order to reduce lateral conduction of heat within the recording material the said overall exposure is preferably very short. Preferably the exposure is not more than 10- second in duration and the best results are obtained withexposures of less than 10- second, e.g. between 10- and 10- second. Such brief exposure times imply the use of high energy radiation sources. Preferably the intensity of the light incident upon the recording material is at least 0.5 watt.sec./cm.

When using very brief exposure times as above specified a sharp hydrophilic-hydrophobic diflFerentiation can be obtained.

The shorter the exposure time for a same amount of light energy used in the epxosure the higher the local increase of temperature.

Various types of flash lamps are ideally suitable radiation sources for briefly emitting electromagnetic radiation (infra-red, visible and/or ultraviolet light). Particularly useful are gas discharge lamps emitting light substantially in the wavelength range of 0.3,u. to 1.211.. A flash-exposure device particularly suited for application in the present invention is described in the United Kingdom patent application 28,467/ 67 filed June 20, 1967 by Gevaert-Agfa N.V.

Although for reason of extreme copying sharpness a high intensity short duration exposure is applied for heating the silver image and the corresponding areas of the recording layer, a common infrared radiation exposure e.g. in a commercial thermographic copying apparatus e.g. a Thermofax-copying apparatus (registered trade name) yields very satisfying results.

According to a preferred embodiment of the present invention a printing master suited for offset-printing with a lipophilic planographic printing ink is prepared. For that purpose the recording material is image-wise exposed to light to which the silver halide emulsion is sensitive and developed in such a way that a legible directpositive silver image is obtained from a positive original.

The image-wise exposure for producing a legible directpositive silver image in a silver halide emulsion of the type described in the present invention can be a contact exposure through a transparent original or a projection exposure effected by means of an optical system in a camera, an optical enlarger, or an epidiascope.

According to another embodiment a printing form is prepared which is adapted for the production of positive prints of a negative original by reverse planographic printing, i.e. with a printing ink containing an aqueous coloured phase. Therefor, the recording material is imagewise exposed to or through a negative original. After the complete processing a printing master is obtained, the hydrophobic parts of which correspond with the image parts of the negative original and the hydrophilic parts of which are the positive printing parts when applying reverse planographic printing. The latter embodiment is of interest for the planographic reproduction of positive legible prints starting from e.g. a microfilm negative.

Offset printing is applied when a printing master containing a directly legible printing image is formed whereas direct printing (master immediately contacting the printing paper) is applied when the master contains a laterally reversed printing image A printing ink which is particularly suited for reversed planographic printing is described and claimed in the Belgian patent specification 676,898 filed Feb. 23, 1966 by Gevaert-Agfa 'N.V. Said ink is defined as a hydrophilic printing ink composition having dispersed therein a lipophili'c phase which is colourless or substantially colourless or of a colour tone contrasting with that of the hydrophilic phase which is coloured e.g. with a black pigment.

A hydrophilic ink for use with a said printing master is not necessarily mainly composed of water. In thecoloured hydrophilic phase hydrophilic water-soluble compounds may be present in a relatively high amount e.g. up to by weight in respect of water, for providing a higher viscosity and a more paste-like consistency to the ink. Hydrophilic compounds, which are particularly useful in this respect; are water-soluble polyols, e.g. ethylene glycol and water-soluble polyoxyalkylene compounds. Lipophilic planographic printing inks are well known so that their composition need not be described here.

The following examples illustrate the present invention without, however, limiting it thereto.

EXAMPLE 1 A gelatin silver bromide emulsion that mainly formed internal latent image and little external latent image was prepared through conversion of a silver chloride emulsion by very slowly adding to said emulsion whilst stirring a 35% aqueous potassium bromide solution in an amount that was considerably higher than the amount theoretically necessary for converting all the silver chloride. The temperature of the emulsion was then kept at 60 C. for 30 minutes.

After the emulsion has been chilled and allowed to gel for 6 hours the emulsion was noodled. The noodles were washed for 1 hour with water (10 litres of water per minute).

By heating the washed noodles, again a liquid silver halide emulsion was obtained (ratio of gelatin/silver nitrate is 0.82). Per kg. of said liquid emulsion the following series of solutions were added:

Solution in ethanol of the following sensitizer (e.g. per 1000 ccs. of solution) S O C-CH=CHCH=G CH N -on, Br- Q 35 cos.

Ccs.

one in isopropanol (5 g. per ccs. of solution) 8 Aqueous solution of potassium iodide (5 g. per 100 ccs. of solution) 6 Aqueous solution of poly(N-vinyl pyrrolidone) (mole weight about 40,000) (20 g. per 100 ccs. of solution) 50 The thus obtained silver halide emulsion was mixed in a ratio by weight of 3:1 with a 40% colloidal aqueous dispersion of polyethylene having a particle size of less than 0.1 and an average molecular weight varying between 15,000 and 30,000.

The ratio by weight of the gelatin of the silver halide emulsion in respect of the latex particles was 2 to 5.

To 1000 cos. of the composition obtained g. of a 4% aqueous formaldehyde solution and 50 g. of a 5% aqueous solution of Tergitol 4 (wetting agent) were added (Tergitol 4 is the sodium sulphate derivative of 7-ethyl- 2-methyl-un-decanol-4 sold by Union Carbide Corporation, and Tergitol is a registered trademark).

The whole composition was coated on a paper base, which was impermeabilized with a cellulose nitrate layer. An amount of silver halide corresponding with 1 g. of silver was applied per sq. /m.

The recording material thus obtained was exposed to an opaque line original in a camera without optical enlargement.

13 Then it was developed for 2 min. at 20 C. in a surface developer of the following composition:

30 see. after the development started the light-sensitive material was overall exposed for the remaining time of the development through a grey filter having a density of 2.7 by means of a 15 watt lamp placed at a distance of 70 cm. from the light-sensitive material. A direct-positive image was obtained from the original.

Within 15 seconds after its development and after having been passed through squeezing rollers the material was overall exposed by means of a rod-like xenon flash lamp axially positioned in a glass cylinder having a diameter of 8 cm., around which cylinder the heat-sensitive surface layer of the developed material was directed to the lamp.

The exposure time was 1250 microseconds and the exposure energy 0.66 watt. sec. per cm.

The thus obtained planographic printing master containing a laterally reversed hydrophobic printing image was then braced on an offset-printing machine printing with a lipophilic printing ink.

EXAMPLE 2 Example 1 was repeated except for the way of developing the direct-positive silver image. The development was carried out at 20 C. for 15 seconds in the surface developer described in Example 1 but whereto 9 g. of carbazic acid were added as fogging agent. The pH of the developer was adjusted to 12 with the aid of sodium hydroxide. The overall exposure step during development was omitted. A direct-positive image was obtained from the original. Without previous fixing, rinsing and drying the recording material was exposed to infrared light in a commercial Thermofax copying apparatus within seconds after its development.

A planographic printing master suited for offset-printing was obtained.

We claim:

1. The preparation of a plnaographic printing master comprising the steps of (a) image-wise exposing to light a recording material comprising a layer formed of a dispersion in a common hydrophilic binder of light-sensitive silverhalide grains of the internal type and an aqueous dispersion of hydrophobic thermoplastic particles which are solid at room temperature,

(b) photographically developing the exposed silver halide grains with a photographic developing agent to obtain in said layer a photographic direct-positive silver image having a density at least 0.1 above the density in the non-image areas, and

(c) uniformly exposing the recording material to electromagnetic radiation which is absorbed by the silver image and transformed into a heat pattern corresponding to said image, said heat pattern having a suflicient intensity to produce in the layer an imagewise hydrophilic-hydrophobic differentiation necessary for making possible planographic printing.

2. The preparation of a planographic printing master according to claim 1, wherein the photographic directpositive silver image is produced by developing the image- 14 wise exposed recording material with an energetic surface developing liquid and uniformly exposing the recording material during such development to actinic light of low intensity.

3. The preparation of a planographic printing master according to claim 1, wherein the photographic directpositive silver image is produced by developing the imagewise exposed recording material in an energetic surface developing liquid and wherein hydrazine, carbazic acid, bicarbamic acid, and/or at least one water-soluble salt thereof has been added to this developer and/or to another processing liquid with which the recording material is Wetted before development.

4. The preparation of a planographic printing master according to claim 1, wherein at least a part of the photographic agent developing is incorporated into the recording material in effective contact with the silver halide grains.

5. The preparation of a planographic printing master according to claim 1, wherein a compound selected from the group consisting of compounds setting free iodide or bromide ions in an aqueous medium is incorporated into the recording material in effective contact with the lightsensitivesilver halide grains.

6. The preparation of a planographic printing master according to claim 1, wherein the recording material comprises a fog-inhibiting compound in effective contact with the silver halide grains.

7. The preparation of a planographic printing master according to claim 1, wherein the developing step of the direct-positive silver image is immediately followed by a stop bath treatment before the overall exposure step (c) is carried out.

8. The preparation of a planographic printing master according to claim 1, wherein the recording material is at most damp at the moment said overall exposure of the recording material containing the direct-positive silver image is carried out.

9. The preparation of a planographic printing master according to claim 1, wherein the dispersed polymer particles range in size from 0.01 to 50p.

10. The preparation of a planographic printing master according to claim 1, wherein said hydrophobic thermoplastic particles are present in a ratio by weight greater than 1:1 with respect to the hydrophilic binder.

11. The preparation of a planographic printing master according to claim 1, wherein the uniform exposure for generating heat in the developed silver image is carried out by means of a flash lamp.

12. The preparation of a planographic printing master according to claim 1, wherein the uniform exposure for generating heat in the developed silver image is carried out by means of an infrared lamp.

References Cited UNITED STATES PATENTS 2,456,953 12/1948 Knott et a1. 96-64 2,456,956 12/1948' Knott et al. 96-'64 2,503,758 4/1950 Murray 96-33 3,284,197 11/1966 Smith 96-33 3,476,937 11/ 1969 Vranken 250-65 NORMAN G. TORCHIN, Primary Examiner A. T. S. PICO, Assistant Examiner US. Cl. X.R.

96-27 R, 48 HD 

